1. Field of the Invention
The present invention pertains to an apparatus and method which ensure substantially even and repeatable heat transfer to and from the entire substrate used in semiconductor processing. In particular, the platform upon which the substrate sets during processing is designed to permit heat transfer to the very edge of the substrate so that minimal substrate surface space is unavailable for processing.
2. Description of the Background Art
In the manufacture of many electronic or electrical components such as integrated circuits, there is a need to deposit thin films on substrates. Materials such as, for example, aluminum, titanium, tungsten, tantalum, tantalum nitride, cobalt, and silica may be deposited on ceramic, glass or silicon-derived substrates using physical vapor deposition (PVD) processes such as a sputtering process. Another method of deposition of solid materials upon a substrate is chemical vapor deposition (CVD), wherein a solid material is deposited from a gaseous phase onto a substrate by means of a chemical reaction. The deposition reaction involved is generally thermal decomposition, chemical oxidation, or chemical reduction. The CVD process can be used to deposit many elements and alloys as well as compounds including oxides, nitrides and carbides.
The thin film deposited is subsequently etched or otherwise fabricated into the circuits and/or components.
In a typical PVD process, such as sputtering, a low pressure atmosphere of an ionizable gas such as argon or helium is produced in a vacuum chamber. The pressure in the vacuum chamber is reduced to about 10.sup.-6 to 10.sup.-10 Torr, after which argon, for example, is introduced to produce an argon partial pressure ranging between about 0.0001 Torr (0.1 mTorr) and about 0.020 Torr (20 mTorr). Two electrodes, a cathode and an anode, are generally disposed in the vacuum chamber. The cathode is typically made of the material to be deposited or sputtered, and the anode is generally formed by the enclosure (particular walls of the vacuum chamber, or the platform upon which the substrate sits, for example). At times, an auxiliary anode may be used or the article to be coated may serve as the anode. Typically a high voltage is applied between these two electrodes, and the substrate to be coated is disposed upon a platform positioned opposite the cathode. The platform upon which the substrate sets is often heated and/or cooled, and heat is transferred between 8 the platform and the substrate, to assist in obtaining the desired coating upon the substrate. The coating is a thin film which needs to have an even thickness, controlled stress and the desired material morphology, while providing step coverage. To obtain such a thin film coating, it is desirable to maintain the substrate at a uniform temperature within a few .degree.C.; preferably, the temperature is near but below the melting point of the material from which the film is being formed. It is very important that the substrate temperature be repeatable each time a given process is carried out. Thus, the heat transfer between the platform and the substrate must be uniform and repeatable.
In a typical CVD process, to facilitate uniformity of deposition coverage of the substrate, the deposition is carried out in a vacuum chamber under a partial vacuum. The pressure in the CVD chamber commonly ranges between about 0.070 Torr (for plasma enhanced CVD) to about 200 Torr (for "high pressure" CVD). As reactive gases are fed into the chamber, they are directed via pressure differential (created by the vacuum system applied to the CVD chamber) across the surface of the substrate to be coated, in a manner which provides an even flow of reactants over the substrate surface. The deposition is also controlled by the temperature of the surfaces which contact the reactant gases. Thus, it is critical that the substrate surface be controlled at a desired uniform temperature.
The platform upon which the substrate sets is commonly used as the means for transferring heat to and from the substrate. Radiant, inductive, or resistance heating is commonly used to heat a support platform; it is also possible to circulate a heat transfer fluid internal to the support platform to provide heating or cooling of the support platform.
When the pressure in the process (vacuum) chamber is about 5 Torr or less, convective/conductive heat transfer between the substrate and the platform becomes impractical. Since the substrate and the platform typically do not have the perfectly level surfaces which would enable sufficiently even heat transfer by direct conduction, it is helpful to provide a heat transfer fluid between the platform and the substrate, to assist in providing even heat transfer between the support platform and the substrate. Preferably the heat transfer fluid used between the substrate and its support platform is in constant movement (flowing) to provide yet another improvement in the uniformity of the heat transfer. The heat transfer fluid commonly used between the support platform and the substrate is one of the gases used in the sputtering, CVD or etch process.
A frequently-used substrate support platform design is one having a substrate contact surface which is principally flat-faced with openings and/or exposed channels spaced at various locations upon this surface. The fluid used to transfer heat between the platform and the substrate flows through the openings or is fed through an opening into exposed channels in the flat-faced surface of the platform. The heat transfer fluid can be provided to the platform from a fluid supply source via tubing which connects the fluid supply source to the platform. The platform itself may include various means for directing fluid to the openings and/or exposed channels in the surface of the platform.
In operation of a semiconductor processing apparatus, the substrate to be processed is typically mechanically or electrostatically clamped along its edges to the substrate-facing surface of the support platform. The fluid used to transfer heat between the support platform and the substrate is typically a gas such as helium, argon, hydrogen, carbon tetrafluoride, or hexafluoroethane, for example, or other suitable gas that is a good heat conductor at low pressure. This heat transfer fluid is applied through openings or supplied into exposed channels in the substrate-facing surface of the support platform. Presence of the heat transfer fluid between the substrate and support platform surface establishes a nearly static gas pressure which commonly ranges from about 1 Torr to about 100 Torr, depending on the particular film deposition/etch process. It is preferable that the pressure between the substrate and its support platform be designed to exceed the overall pressure in the process chamber so that a minor, constant net flow of fluid from beneath the substrate into the process (vacuum) chamber occurs. This helps avoid contamination of the support platform and fluid supply system.
The positive pressure between the back (non-processed) side of the substrate and its support platform, as described above, tends to bow a thin substrate which is mechanically clamped at its edge. Bowing of the substrate reduces the amount of heat transfer near the center of the substrate and results in uneven heating of the substrate in general.
Once the processing steps have been completed and it is desired to remove the semiconductor-processed substrate from the support platform, the clamping means is released and lift fingers are used to raise the semiconductor-processed substrate above the support platform so that handling means can grasp it and move it away from the support platform. These lift fingers have been placed exterior to the substrate support platform so the fluid flowing across the back side of the substrate and across the support platform will not flow down an opening through which a lift finger is operated, but will flow with even distribution across the substrate support platform.
The combination of the support platform, clamping means and externally-located lift fingers (as described above) has the disadvantage that the edge of the substrate, which constantly loses heat due to radiation, does not receive adequate heat input from the support platform. The uneven heat transfer gas pressure at the exterior edges of the substrate results in a substrate which is colder around its exterior edge, so that this portion of the substrate cannot be used for semiconductor device construction. Due to the importance of using all of the "real estate" surface of the substrate, it would be highly desirable to provide even heating of the semiconductor substrate to the very edge of the substrate.